Ammeters can be used to measure the current consumption of an integrated circuit CPU. Traditionally, a precision resistor is used in the power supply grid and the voltage drop across the precision resistor is measured. Using the known precision resistor value and the measured voltage drop, the CPU's current draw and power level can be calculated. The introduction of the precision resistor into the power supply grid wastes power since the precision resistor is always there even when the CPU's current draw is not being measured.
It is important that the precision resistor have a high level of stability across a range of temperatures and currents so that accurate measurements can be made. The use of such a precision resistor adds to the overall costs of the CPU. It is also important that the voltmeter that is utilized to measure the voltage drop be initially calibrated and stable across a range of temperatures.